Vehicle drive trains known from practice are to an increasing extent constructed with automatic transmissions having frictional shift elements such as disk clutches or disk brakes. When a command is given for a gear ratio change in the automatic transmission, at least one frictional shift element engaged in the force flow of the automatic transmission in order to produce the gear ratio currently engaged in the automatic transmission has to be disengaged from the force flow of the automatic transmission, and at least one other frictional shift element, which is disengaged from the force flow of the automatic transmission while the current gear ratio is engaged, has to be engaged in the force flow of the automatic transmission in order to obtain the required gear ratio.
During this the torque transmitted by the frictional shift element engaged in the force flow in order to obtain the current gear ratio is taken up more and more as shifting time increases by the frictional shift element that has to be engaged in the force flow of the automatic transmission in order to produce the required gear ratio, while the torque that can be transmitted by the shift element to be disengaged decreases.
Disadvantageously, frictional shift elements in the disengaged operating condition give rise to drag torques which impair the overall efficiency of an automatic transmission to an undesired extent.
For this reason, besides frictional shift elements, automatic transmissions are also made with positively interlocking shift elements in the area of which no drag torques that affect the overall efficiency of an automatic transmission adversely occur.
However, only when they are close to their synchronous point can interlocking shift elements be shifted from a disengaged operating condition in which no torque can be transmitted by the interlocking shift element, to their engaged operating condition in which the whole of the torque applied can be transmitted by the interlocking shift element. Furthermore, interlocking shift elements engaged in the force flow of a transmission device can only be disengaged from the force flow with small shifting forces provided that they are close to their load-free operating condition. Both for the synchronization of interlocking shift elements and to bring them to their load-free condition, in contrast to frictional shift elements, additional structural devices are needed in order to be able to carry out shift operations that involve at least one interlocking shift element, within desirable shifting times.
Such additional structural devices, however, undesirably increase both the manufacturing costs of a transmission device and the structural space that it occupies.